2.2 Measuring Earthquakes

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Vocabulary

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Focus-point beneath Earth's surface where rock breaks under stress and causes an earthquake
Epicenter-the point on Earth's surface directly above an earthquake's focus
Seismic wave-vibration that travels through Earth carrying the energy released during an earthquake
P wave-type of seismic wave that compresses and expands the ground
S wave-type of seismic wave that moves the ground up and down or side to side
Surface wave-when P and S waves reach the surface some are transformed into these
Seismograph-device that records ground movements caused by seismic waves as they move through Earth
Magnitude-measurement of an earthquake's strength based on seismic waves and movement along faults
Mercalli Scale-a scale that rates earthquakes according to their intensity and how much damage they cause
Richter Scale- a scale that provides accurate measurements for small nearby earthquakes
Moment magnitude scale- scale that rates earthquakes by estimating the total energy released by an earthquake

Outline:

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Summary

• earthquakes always begin in rock below surface

Seismic Waves

• carry the energy of an earthquake away from the focus through Earth's interior, and across the surface
• three categories:
• Primary Waves
  • a.k.a. P Waves
  • causes buildings to contract and expand
  • travels through both solid and liquid
• Secondary Waves
  • a.k.a. S waves
  • when reached surface, it shakes structures violently
  • cannot travel through liquid but can travel through solid
• Surface Waves
  • when P and S waves both reach the surface, some of them transforms into Surface Waves

Detecting Seismic Waves

• a mechanical seismograph includes a heavy weight attached to a frame by a spring or wire
• the pen (connected to the weight) rests its point on a rotating drum
• the electronic seismograph converts ground movements into a signal that can be recorded and printed

Measuring Earthquakes

• at least 20 different measures for rating an earthquake
• the Mercalli Scale
  • rate earthquakes according to its intensity
  • earthquake's intensity is the strength of its ground motion in a given place
  • same earthquakes can have different ratings because it causes different amounts of damage at different places
  • the Mercalli Scale is developed early in the twentieth century
• Richter Scale
  • developed in 1930s
  • provides accurate measurements for small, nearby earthquakes
  • doesn't work very well if the earthquakes are large and/or far away
• Moment Magnitude Scale
  • can be used to rate earthquakes of all sizes near or far

Locating the Epicenter

• use seismic waves to locate/find the epicenter
• seismic waves travel at different speeds
• P waves arrives before the S waves
• scientists measure the difference between the arrival time of P waves and S waves to tell how far away the epicenter is from the seismograph
• the farther away an earthquake is, the greater the time between the arrival of P and S waves
• geologists draw at least 3 circles
• center of each circle is the seismograph's location
• radius of each circle is the distance from epicenter
• point where each circle intersect is epicenter

Diagram

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